1. Field of the Invention
The present invention relates to a radar apparatus that calculates target information based on a reflection signal which is reflected from an object to be detected (hereinafter referred to as a “target”) and which is received by a plurality of receiving elements when a transmitting signal (radio wave) is radiated to the target.
2. Description of the Related Art
Conventionally, as a radar apparatus capable of calculating a distance and a relative speed to a target, there has been known a frequency modulation radar apparatus that sends a transmitting signal with its frequency modulated to the target in a continuous or successive manner, receives a reflection signal from the target, and calculates the distance and the relative speed with respect to the target based on the reflection signal thus received.
As a target direction calculation method in such a kind of radar apparatus, there has been known a method for calculating the direction of a target by scanning a transmitting signal while swinging a transmitting unit in a mechanical manner. In addition, there has also been known a digital beam forming (Digital Beam Forming, hereinafter being referred to as “DBF”) synthetic processing that calculates the direction of a target by outputting a transmitting signal without mechanically swinging a transmitting unit, and by performing digital signal processing on reception signals which are received by an antenna in the form of an array having a plurality of channels (see, for example, a first patent document: Japanese patent application laid-open No. H11-133142).
The DBF synthetic processing is a method of calculating the direction of a target by producing an angular spectrum from the received signals that are obtained from individual channels of an array antenna comprising a plurality of channels, and detecting a peak of the angular spectrum.
In particular, in the technique disclosed in the first patent document, an angular spectrum is calculated by performing the DBF synthetic processing on the signals received by the frequency modulation radar apparatus, and the direction of the target is calculated based on the angular spectrum thus obtained.
In the technique described in the first patent document, the DBF synthetic processing is not performed on all the frequencies of the received signals, but only on the frequencies, of which the amplitude is greater than a predetermined value, in the beat frequency spectrum that is obtained by applying a frequency analysis to the received signals, whereby the amount of calculations to obtain the direction of the target can be reduced.
In addition, super-resolution direction-of-arrival estimation processing such as a MUSIC (Multiple Signal Classification) method, etc., is known as a method for calculating the direction of a target by outputting a transmitting signal to a target without mechanically swinging a transmitting unit, and by performing digital signal processing on signals that are reflected from the target and received by an array antenna having a plurality of channels (see, for example, the following first non-patent document).
R. O. Schmidt, “Multiple Emitter Location and Signal Parameter Estimation”, IEEE Trans. AP-34, No. 3, pp. 276-280 (1986)
The MUSIC method is a method of calculating a covariance matrix, performing an eigenvalue decomposition of the covariance matrix so as to obtain eigenvectors, calculating an angular spectrum from the eigenvectors, and calculating the direction of a target from the angular spectrum.
In the beat frequency spectrum before the DBF synthetic processing in the conventional radar apparatus, there has been a problem that when the RCS (Radar Cross Section) of a target is small or a target is at a distance, it becomes difficult to detect a peak resulting from the target.
More specifically, according to the method of performing the DBF synthetic processing only on the frequencies, of which the amplitude is greater than the predetermined value, in the beat frequency spectrum obtained by applying the frequency analysis to the received signals, as in the first patent document, there has been a problem that if the RCS of a target is small or a target is far away, it might be impossible to detect the target.
In addition, in the technique for calculating the direction of the target by using the DBF synthetic processing, as in the first patent document, there is also a problem that the construction of the antenna has to be changed in order to obtain a high degree of resolution.
On the other hand, according to the super-resolution direction-of-arrival estimation processing described in the above-mentioned first non-patent document, a high degree of resolution can be obtained without changing the construction of the antenna, but a great number of calculations is required for calculating a covariance matrix with respect to all the frequencies of the beat frequency spectrum of each antenna, performing an eigenvalue decomposition of the covariance matrix to obtain eigenvectors, and deriving an angular spectrum from the eigenvectors. As a result, there is a problem that the huge amount of calculations is necessary as compared with the case in which only an angular spectrum of limited frequencies is calculated, thus resulting in a substantial increase in the calculation load.